1. Field of the Invention
The present invention relates to hard coatings for tools. More particularly, the present invention relates to diamond coated cemented tungsten carbide articles and methods for making diamond coated cemented tungsten carbide articles.
2. The Prior Art
Polycrystalline diamond (PCD) cutting tools, comprising a piece of polycrystalline diamond fastened to the tip of a tool insert are known in the art. These tools are expensive to manufacture and do not readily lend themselves to indexing for increased tool life. In addition, PCD tooling having complex shapes, i.e., taps, drill bits, cannot be formed using any known techniques. PCD tools are typically run at cutting speeds of around 2,500 SFM when cutting materials such as 390 aluminum.
Numerous attempts have been made to provide diamond coated tools which have performance approaching that of PCD tools because they would be less costly to manufacture and use, and because diamond coated tools having more complex shapes than are possible with PCD tools are theoretically manufacturable employing substrates such as cemented tungsten carbide.
A significant challenge to the developers of diamond-coated tooling is to optimize adhesion between the diamond film and the substrate to which it is applied, while retaining sufficient surface toughness in the finished product. Substrates like Si.sub.3 N.sub.4, and SiAlON can only be formed into a few geometries, limiting their commercial potential. Sintered tungsten carbide (WC) substrates without cobalt or other binders have been studied but can be too brittle to perform satisfactorily as tooling in machining applications.
Cemented tungsten carbide substrates incorporating a cobalt binder in concentrations between about 4% and 6% (WC/Co) have the requisite toughness and thus show the greatest long-term commercial promise for tooling applications. A cemented tungsten carbide substrate with up to 6% cobalt would provide adequate surface toughness for most machining tasks. Cemented tungsten carbide can be formed into a variety of geometries, making it a potential material for drilling operations, die manufacturing, and other applications of value to the automobile and other industries. It is therefore desireable to provide a way to coat cemented tungsten carbide substrates with a layer of diamond film having adequate adhesion to the substrate for use as a machine tool.
It has been reported in the literature that the use of a cobalt binder in cemented carbides inhibits adhesion of the diamond film to the substrate. R. Haubner and B. Lux, Influence of the Cobalt Content in Hot-Pressed Cemented Carbides on the Deposition of Low-Pressure Diamond Layers, Journal De Physique, Colloque C5, supplement au no. 5, pp. c5-169-156, Toma 50, May 1989. Indeed, conventional wisdom indicates that successful use of cemented tungsten carbide substrates may only be achieved by utilizing substrates containing no cobalt, as taught in U.S. Pat. No. 4,990,403; no more than 4% Co binder, as taught in U.S. Pat. No. 4,731,296, or by deliberately depleting the cobalt concentration at the surface of the substrate. It is known to deplete the cobalt concentration at the surface of the substrate by selective etching or other methods, M. Yagi, Cutting Performance of Diamond Deposited Tool For Al 18 mass % Si Alloy, Abst. of 1st Int. Conf. on the New Diamond Sci. & Technol., pp. 158-159, Japan New Diamond Forum, 1988., but this decreases the surface toughness of the substrate and can cause chipping of the substrate and applied diamond film. Increased adhesion of diamond to the substrate may be achieved by decarburizing the substrate prior to deposition, as taught in European Patent Application Publication No. 0 384 011, but use of this procedure does not optimize substrate toughness and does not lend itself well to manufacturing environments where repeatability and consistency are important issues.
The prior art teaches polishing or scratching the surface of a cemented tungsten carbide substrate prior to attempting diamond deposition due to the enhancement to the nucleation process caused by scratching and polishing. Haubner and Lux; Yagi; M. Murakawa et al., Chemical Vapour Deposition of A Diamond Coating Onto A Tungsten carbide Tool Using Ethanol, Surface Coatings Technology, Vol. 36, pp. 303-310, 1988; Kuo et al., Adhesion and Tribological Properties of Diamond Films on various substrates, J. Mat. Res., Vol. 5, No. 11, Nov. 1990, pp. 2515-2523. These articles either teach use of polished substrates or indicate poor results obtained by utilizing substrates whose surfaces have not been prepared by polishing or scratching.
A promising solution to the adhesion problem has been to employ an interlayer between the diamond and a WC/Co substrate. This encapsulates the Co, optimizing adhesion while allowing the substrate to retain its toughness. It may also be possible to choose an effective interlayer material that bonds strongly to diamond, further increasing adhesion. U.S. Pat. No. 4,707,384 discloses use of a titanium carbide interlayer. U.S. Pat. Nos. 4,998,421 and 4,992,082 disclose utilization of a plurality of layers of separated diamond or diamond like particles interposed with layers of a planarized bonding material.
It would, however, be advantageous to develop a direct diamond coated cemented tungsten carbide article having superior adhesion for machining purposes. A process for producing such a diamond-coated cemented tungsten carbide article would also be desireable.